Our aim is to study the distribution of nicotinic acetylcholine receptors in intact and cultured tissues of the peripheral and central nervous system in relationship to the development and function of synapses. To this purpose histochemical localization of alpha-bungarotoxin bound to the receptors is used in conjunction with light and electron microscopy. In the past year we have used an alpha-bungarotoxin-horseradish peroxidase conjugate to study the formation of cholinergic synapses in developing chick retina, and have studied the control of nicotinic acetylcholine receptor aggregation on cultured skeletal muscle cells by a macromolecular factor secreted by neuroblastoma-glioma hybrid cells.